I think they go the Inhumans route. In A:AOU, they could easily leave out the source of their powers with the old "memory wipe" gimmick. Neither Pietro or Wanda remembers their past. As an aside, it also side-steps the issue of their parenthood quite deftly. Then, when Inhumans arrives in Phase 3 or 4, Wanda and Pietro discover their Inhumanity and as a bonus lend some name recognition to the project.